A transformer is means for changing an alternating voltage utilizing the principle of electromagnetic induction, and has been widely used in the technical field of power electronics. The structure of a transformer in the prior art is illustrated in FIG. 1. The transformer consists of an E shaped magnetic core, an I shaped magnetic core 12, and a transformer winding 13. Meanwhile, the E shaped magnetic core includes a middle leg 111, two side legs 112, and two bottom legs 113. The transformer winding 13 includes a primary winding and a secondary winding. The transformer winding 13 is wound on the middle leg 111 of the E shaped magnetic core. The middle leg 111 of the E shaped magnetic core is of an air gap. The size of the excitation inductance of the transformer can be adjusted by adjusting the width of the air gap.
However, the excitation inductance of the transformer of such structure is relative small, resulting in a relative large excitation current existed in the primary winding of the transformer. The magnetomotive force generated by the excitation current spans the secondary winding, and induces to generate additional eddy current loss in the secondary winding, thereby reducing transform efficiency of the transformer. And, the value of such additional eddy current loss is generally in proportion to the thickness of a copper foil, and therefore it is impossible to improve the transform efficiency of the transformer by increasing the thickness of the copper foil.